A Manual of Field Astronomy (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1916 Excerpt: ...half of the measurements with the telescope direct and half with it inverted. The average of the results is then free from the effect of errors in the line of sight or in the height of standards, and should be of greater precision than any one reading. Vertical angles cannot be repeated; but whenever time permits at least two readings should be taken, one with the telescope direct and one with it inverted, releveling if necessary after reversing the instrument. This, of course, presupposes the use of a transit with a full vertical circle. The mean of the two readings should be free from the effect of errors in adjustment in line of sight, standards, telescope level, and index error. If, because the instrument does not have a full vertical circle or because of lack of time, the above method cannot be used, care must be taken to see that the axis of the telescope level is parallel to the line of sight; and the index error must be determined and proper correction made. The index error is the reading of the vertical arc or circle when the line of sight is horizontal. It may be convenient to remember that for use with vertical angles read above the horizontal, if the zero of the vernier is to the right of the zero of the vertical arc when the telescope is horizontal the index correction is positive; while if the zero of the vernier is to the left of the zero of the vertical arc when the telescope is horizontal the index correction is negative. Concerning the sextant: The sextant is not adapted to the measurement of horizontal angles between objects at different elevations; but vertical angles may often be measured with greater precision with the sextant than with the transit, on account of the greater radius and finer graduation of the limb. There is usually an ...